Combined heat and power systems or CHP systems are a well-accepted technology. Also known as cogeneration, many thousands of examples are installed and operating throughout the world. One of the most common versions uses a gas turbine to drive an electrical generator to produce electricity. At the same time, during this electrical energy generation, when available the heat from the gas turbine exhaust can be utilized to produce thermal energy in the form of steam, hot water or hot air. The thermal energy can be used for a myriad of applications, including comfort heating and domestic water heating as well as for air conditioning using an absorption chiller.
A typical prior art CHP system is shown in FIG. 1. While natural gas is the preferred and most commonly used fuel in CHP systems, and natural gas will be referred to as the fuel in this application, it must be recognized that other fuels may be equally applicable and can be readily used.
Ambient air 10 is compressed in compressor 12 to several atmospheres. The compressed air then enters combustor 14. Natural gas 16 has been compressed by natural gas compressor 18 to a pressure that is high enough to flow through modulating fuel valve 20 and into the elevated pressure in combustor 14 where it burns with the ambient air. The hot combustion gases leave combustor 14 and expand through turbine 22 before going through bypass valve 26 that allows the hot gas turbine exhaust gases to go through heat recovery unit 28 or to bypass it. Bypass valve 26 modulates so that enough hot gases go through heat recovery unit 28 to match the thermal load. The rest of the hot exhaust gases bypass the heat recovery unit 28 and are discharged to atmosphere along with the discharge from heat recovery unit 28.
The turbine 22 drives compressor 12 through shaft 24. It also drives motor/generator 30 through shaft 24, compressor 12, shaft 32, gearbox 34 and shaft 36. Motor/generator 30, such as an induction motor/generator, usually produces 60 Hz. or 50 Hz. power. The electrical output of motor/generator 30 goes through breaker 38 to electric load 40.
Starter motor 42 is powered by battery pack 44 that is charged by battery charger 46. The modulating fuel valve 20 controls the flow of fuel to regulate the electrical output.
In recent years, microturbines have been used in CHP systems. A microturbine is generally defined as a small turbogenerator in which the gas turbine is normally utilized with a permanent magnet rotor rotatably driven within an electrical winding by the gas turbine.
The microturbine CHP system concept is essentially the same but with two significant differences which are shown in prior art FIG. 2. The gearbox 34, shaft 36, generator 30 and starter 42 that were shown in FIG. (1) have been deleted and replaced with motor/generator 50 that turns at the same speed as compressor 12 and turbine 22. As motor/generator 50 turns at a very high rate of speed, it puts out high frequency power. Thus rectifier/inverter 52 is added to rectify the output of motor/generator 50 to direct current and then invert it to a more usable frequency such as 60 Hz. or 50 Hz. As motor/generator 50 is a motor as well as being a generator, it is used as a starter motor and receives its energy from rectifier/inverter 52 that receives its power from battery pack 44 which is kept charged by charger 46.
The other significant difference is that most microturbines use recuperators. FIG. 2 shows recuperator 54 which preheats the air leaving compressor 12 before it goes to combustor 14 by using the heat in the discharge of turbine 22 before it goes to bypass valve 26. This reduces the fuel consumption of the microturbine but also reduces the available thermal energy. Therefore some microturbines used for CHP systems do not have recuperators.
In general, gas turbines are used in those applications where individual units are rated at 500 kW or more, while microturbines are used in applications where individual units are rated from 30 kW to 200 kW.
Maximum efficiency in a CHP system occurs when all of the electrical and thermal energies are used beneficially. Thus, it is very hard to achieve efficient operation, and therefore economic operation, when the loads change rapidly and substantially. Residential applications are particularly difficult as electric loads may spike as various appliances are used. Correspondingly, the loads may drop to close to zero during periods when the sole loads are items such as electric clocks. At the same time, the thermal loads change precipitously as comfort heating and domestic water heating units cycle ON and OFF. Many commercial facilities have these same concerns.
The cost of a CHP system is generally too high to be considered for small residential or commercial loads. Although the rotor groups could easily be derived from turbochargers, which have annual production rates in the millions and are therefore inexpensive, the need for precision controls, starting mechanisms, recuperators, fuel gas compressors and heat recovery units with bypasses drive up the cost.
If the generator is not paralleled with the utility, precision controls are needed to maintain voltage and frequency. If the generator is to be paralleled with the utility, precision controls will be needed to bring the electrical output to the correct voltage, frequency and phase before paralleling. These controls also monitor the loads and match the generator output to the load. A critical need is to control fuel flow during startup so that the generator set will accelerate to its operational speed without overheating. An additional function is to control the bypass on the exhaust heat recovery unit so that the thermal output matches the thermal load.
The starting mechanism generally consists of a starter motor, the appropriate electrical and mechanical devices to engage and disengage the motor, and the source of starting energy that is generally batteries. These batteries usually have an associated charger.
Recuperators tend to be the most expensive single component in gas turbines and microturbines that are so equipped. They transfer heat from the turbine exhaust into the air entering the combustor to reduce fuel consumption and improve efficiency.
The exhaust heat recovery units need bypasses so that the output can be reduced. CHP systems that are not paralleled with the grid operate at electrical power outputs that match the electric load. This also determines the thermal output. When the thermal loads are less than the thermal output, the bypass reduces the output to match the load, thus wasting energy and reducing system efficiency. A typical example of reduced thermal load would be when the thermal output is used for heating or air conditioning and the weather is mild.
The combustors of conventional gas turbines operate at several atmospheres of pressure. If the fuel is natural gas, it must be compressed to a pressure higher than that of the combustor or it will not flow into the combustor. These gas compressors are expensive and tend to be inefficient thus imposing a significant parasitic load on the CHP system and further reducing the efficiency.